The invention relates to a method of transmitting digital information in a time-division multiplex telecommunications network over subscriber lines connecting the digitally operating subscriber sets to a subscriber concentrator.
The digitization of time-division multiplex telecommunications networks, insofar as the subscriber sets are concerned results, among other things, in a simpler structure for the subscriber concentrators. It is, therefore, possible to connect a larger number of subscribers to a terminal exchange than the number of time slots available for the retransmission over time-division multiplex paths.
As a rule, four-wire lines are needed for transmitting digital information, in contradistinction to the transmission in time-division multiplex telecommunications networks in which the units of information appear in the form of pulse-amplitude-modulated pulses. However, this means that additional expenditures are required because of the necessary four-wire construction of the subscriber lines connecting the subscriber sets to the subscriber concentrator. Of course, this additional cost would cancel out the advantage of digitization of the subscriber sets mentioned hereinabove.
Pulse-amplitude-modulated pulses of a time-division multiplex telecommunications network can be transmitted over two-wire lines. This is due to the fact that in the case of simultaneous transmission of information in both directions in the form of a charge exchange between the capacitors contained in interconnected subsets, there is no danger that the units of information will interfere with one another. This is not the case when units of information are represented by means of digital values.
Nevertheless, even in the case of transmission of pulse-amplitude-modulated pulses of a time-division multiplex telecommunications system, it is, likewise, old in the art to carry out the transmission in both directions in a time-interleaved arrangement, because in a prior art switching arrangement for connecting a four-wire line to a two-wire time-division multiplex path (West German Examined Patent Application No. 1,287,161), the samples to be sent in both directions of transmission to the time-division multiplex path are divided into two equal parts and transmitted immediately one after the other, so as to use the time-division multiplex path only for the shortest time possible.
In order to again obtain samples having the original length, pulse stretching elements are inserted into the receiving branches of the four-wire lines connected to the two-wire time-division multiplex path. Apart from the fact that the pulse stretching elements constitute an additional electrical outlay, the reduction of the pulse duration of the pulses transmitted over the two-wire line results in deterioration as far as the damping and distortion factors are concerned which, inasmuch as the time-division multiplex path is the multiplex bus bar of a time-division multiplex exchange, is acceptable because of the short length thereof.
In connection with the transmission over subscriber lines between a concentrator and the subscriber sets which, in certain circumstances, can be as long as 10 kilometers, and in connection with the pulse length that is customary for the transmission of digital information, the bits of a code word used for pulse-code modulation are approximately only half as long as the samples used for pulse-amplitude-modulation. Such halving of the transmission intervals would lead to rather significant deteriorations of the transmission quality.
It is an object of the invention to provide a method for transmitting digital information of a time-division multiplex network over subscriber lines connecting the digitally operating subscriber sets to a subscriber concentrator, which enables one to run the subscriber lines on a two-wire basis, thereby achieving such favorable transmission characteristics that additional components, such as regenerators for the subscriber lines, can be dispensed with.